Just because Buffy is a vampire slayer does not mean she has no other threats to deal with. There are different hellish creatures to stop from terrorizing the human population of Sunnydale. But this is already season three, and it is now past the equator, so "Helpless" comes up with something special. No, it does not introduce a new demon or a new monster, and it does not set up another apocalyptic event. It is about vampires again, only there is something very refreshing at work.
Being the slayer means secretly having extraordinary physical abilities. Buffy has them, but something is wrong this time. This time she doesn't have them for some reason. That reason will be clarified. It is one thing to be a regular human being under the attack of a demonic creature. It is bad enough. At least, it comes out of the blue and, hopefully, might end quickly enough. Buffy, on the other hand, is now in a situation where she has to go and fight a creature way stronger than any human without the special strength she usually needs and uses because she has been somehow drained of that strength. So, she knows what is in store for her, she knows what danger is waiting for her. To make things worse, someone close to her is being used as hostage and bait. It isn't just her own life Buffy is concerned with here.
What is a superhero without their superpower? Only the person they are. Buffy may be just a fragile human girl right now, but she is a caring person, and she is resourceful. These characteristics are hers. And what she goes through in this episode can make one like her and worry about here even more. That's what heroes do. They keep going no matter what. That is why their stories are so impactful and inspirational.